(1) Field of the Invention
The present invention relates to a fluid detector which can differentiate between liquid fluid and foam, bubbles, or surges of the fluid; more particularly, it is to a device generally used to detect a low water condition in a boiler system
(2) Background of the Invention
Boilers have been used for generating steam in radiant heating systems in both residential and commercial applications for a number of years. The systems generally operate by heating boiler water to produce steam. The steam is then distributed through a piping system and radiators to heat the facility. Once distributed to radiators, the resultant steam condenses and returns to the boiler to be heated again and redistributed.
Steam boiler systems can be damaged if they have too much water or if foaming occurs within the boiler. In either case, since steam exits the boiler at high velocities, it can propel liquid water from the boiler into the piping system. This condition, known to the industry as priming, occurs when the high-velocity steam drives slugs of water through the piping system and equipment. When these slugs of water impact heating system components and piping, it causes water hammer which can lead to damage of piping, valves, or heating system components such as radiators. When foaming occurs in a steam boiler, priming becomes a more frequent condition. For these reasons, boiler systems are filled to their desired water level during installation and steps, such as cleaning the boiler and piping or chemical treatments, are taken to control foaming in the boiler.
Because of the way these boiler systems operate, it is also necessary that there be sufficient water in the boiler system at all times. If the water level drops too low, the water in the boiler can flash to steam explosively, seriously injuring or killing people or damaging the boiler, facility or both. Boilers generally include a Low Water Cutoff (LWCO) which will serve to turn off the heat source for the boiler if the water level drops below a predetermined safe level. The LWCO therefore serves as a defense against the system being operated with insufficient water and indicates when additional water needs to be added.
There are many types of different systems used in an LWCO to detect the low water condition. In the simplest example, a simple float switch may be used. Historically, these float style LWCO devices function reliably, so long as they receive proper daily or weekly maintenance like blowdown to remove deposits that can cause moving parts to get stuck in a position that falsely indicates a proper water level. Since maintenance is often not performed regularly or sometimes not performed at all, mechanical type LWCO devices are, therefore, subject to improper operation on a regular basis. For this reason, the industry has adopted more sophisticated electronic probe style LWCO devices which have no moving parts.
Probe style LWCO devices generally utilize the electrical conductivity of a fluid, in this case the boiler water, to complete an electrical circuit between two conductors of the probe. One conductor is placed low down in the tank so as always to be in fluid while the other conductor is placed at the desired minimum operating level of water. When both conductors are underwater, electric current can pass through the water between the two conductors completing the electric circuit between them and indicating a safe water level. When the level drops below the level of the upper conductor, the electric circuit is broken which indicates a low water condition.
When the fluid level is stable in the vessel and the surface line is well defined, this type of technology works quite well. In steam boiler applications, however, the stability of the water level and the definition of the water line can degrade during operation. For instance, boiling water will often surge due to heat underneath creating bubbles which pass through the water disrupting the water line. Further, differences in heat throughout the boiler water can further cause water movement which can disrupt the water line making a constantly changing water level. Under intense boiling conditions, the water line becomes very undefined, making it difficult to ascertain the actual water level.
Steam boilers also act under pressure. When the boiler has reached a desired operating pressure, it enters a relatively stable state where steam can be removed and the water boils in a fairly controlled fashion. When heating loads increase rapidly, the additional load on the boiler can result in much more steam suddenly being removed from the boiler, decreasing the pressure in the boiler. This can result in the water being heated more aggressively and boiling more violently as the pressure falls off. A violent boil of water will generally lead to a significant disruption of the water line due to splashing of the water inside the fluid vessel. Further, water which is not completely clean can generate a foam which can float on the surface of the water or even fill the inside of the boiler.
Traditional LWCO technologies generally determine the water level by examining the resistance between the two probes as a method to determine if the conductive fluid is between the probes. The strength of the LWCO signal transmitted via the boiler water is related to the amount of surface area of the probe in contact with the liquid. Therefore, in a well behaved system, as the water line slowly drops across the surface of the upper probe, the signal will decrease as resistance effectively increases. In the boiler system, however, the presence of steam bubbles within the water and an imprecise water line can lead to false readings as the surface area of the probe actually in contact with liquid water can change from instant to instant, even when the water line is significantly above the probe.
In order to prevent false indications of a low water level due to the changes in probe signal levels due to surging or bubbling, traditional low water detectors average the probe signal over a period of time, and then use the average to determine whether there has been a sufficient change over time to indicate a low water level. In the event of small changes to the average over a significant time period, the low water detector will often adjust to accept new values as the expected signal levels (or baseline) and will only trigger a low water condition in the event of a major change from that baseline. These types of systems effectively compensate for small changes in the conductivity such as through alteration of the water chemistry from interaction with the pipes.
The problem with averaging systems is that having their probe in foam or under relatively violent splashing will generally still produce an average probe signal within the period that is similar to the average probe signal when liquid is covering the probe. The alternative problem is, however, that not using the average produces a large number of false indications for low water as the water surges and bubbles during the boiling process but is still at a sufficient operating level.
To attempt to compensate for the false low water indications produced by dynamic water levels, while preventing premature restoration from a low water condition, low water cutoff devices often incorporate a delay feature that requires water to be continuously present before a low water condition is removed or continuously absent for a predetermined length of time before a low water condition is indicated. In some other systems, the low water detection system attempts to correct for dynamic effects by periodically shutting off the boiler's burner circuit and keeping it off for a predetermined duration or settling period. At the end of this period, it is presumed that the dynamic system has stabilized or settled and therefore the low water detection device can take a more accurate reading of whether it is within fluid or not.
While both these options help with the problem of inaccurate detection, numerous issues plague the methods. Requiring continuous low water indications before triggering a low water condition can often make a low water condition less detectable by making it harder to detect foam or surge which will result in less detection accuracy. Further, during a settling period, foam and steam also have an opportunity to condense and return to the boiler, replenishing the water level and possibly altering the determination. Alternatively, a heavy foam may not recondense, leaving the probe still within the foam at the end of the settling period and having the probe still generate an inaccurate reading. The short cycling of the heating cycle created by periodically turning the burner off can also cause numerous problems that lead to premature failures of the heating system resulting in potentially high repair or replacement costs. Further, the system will not be able to meet demands for heat when the shut-off period is implemented.